default:
		return nil, nil // Not a PAX format GNU sparse file.
	}
	hdr.Format.mayOnlyBe(FormatPAX)

	// Update hdr from GNU sparse PAX headers.
	if name := hdr.PAXRecords[paxGNUSparseName]; name != "" {
		hdr.Name = name
	}
	size := hdr.PAXRecords[paxGNUSparseSize]
	if size == "" {
		size = hdr.PAXRecords[paxGNUSparseRealSize]
	}
	if size != "" {
		n, err := strconv.ParseInt(size, 10, 64)